It has been done that an injection needle is introduced into a body cavity through an instrument, such as a channel, of an endoscope and a liquid medicine is injected into a tissue region, such as an organ or a blood vessel, in a body cavity by the injection needle. This type of injection needle has a double sheath structure in which an inner tube having a distal end provided with an injection needle body is inserted in an outer sheath, as disclosed in JP-A 2001-58006 (KOKAI).
When using this type of injection needle, the following procedure is carried out. First, a syringe filled with a liquid medicine is attached to a mouthpiece on a proximal end of the inner tube. Then, the outer sheath in this state is inserted into the channel of the endoscope to make the distal end of the outer sheath project from the distal end of the endoscope. Then, the inner tube is pushed inward to project out the injection needle body from the distal end of the outer sheath and to insert the injection needle body into the tissue region. Finally, the syringe is pushed to inject the liquid medicine into the tissue region.